Hot working method of producing cubeon edge oriented silicon iron from cast slabs

ABSTRACT

A PROCESS FOR THE PRODUCTION OF CUBE-ON-EDGE SILICONIRON SHEET HAVING UNIFORMLY GOOD MAGNETIC PROPERTIES, COMPRISING MELTING, REFINING, CASTING INTO SLABS, HOT REDUCING TO HOT ROLLED THIN BAND, REMOVING THE HOT MILL SCALE, COLD REDUCING TO FINAL THICKNESS IN ONE OR MORE STAGES, DECARBURIZING AND RECRYSTALLIZING IN A WET HYDROGEN ANNEAL, COATING WITH A SEPARATOR AND ANNEALING IN HYDROGEN ABOVE ABOUT 1100*C. THE CAST SLABS ARE INITIALLY HOT REDUCED AT LEAST 5% AT A TEMPERATURE OF 750* TO 1250*C., THEN HEATED ABOVE 1350* C. AND HOT ROLLED TO A THICKNESS OF 2.5 MM OR LESS. THE INITIAL HOT REDUCTION PRODUCES A STRUCTURE WHICH PREVENTS EXCESSIVE GRAIN GROWTH DURING THE SUBSEQUENT HEATING AND HOT ROLLING STEPS.

Oct. 9, 1913 M. F. LITTMANN HOT-WORKING METHOD OF PRODUCING CUBE-ON-EDGE'ORIENTED SILICON-IRON. FROM CAST SLABS Filed Nov. 4, 1971 INVENTGR ATTORNEYS Oct. 9, 1973 M,- LlTTMANN 3,764,406

HOT-WORKING METHOD OF PRODUCING CUBE-ON-EDGE ORIENTED SILICON-IRON FROM CAST SLABS Filed Nov. 4, 1971 2 Sheets-Sheet 2 l NVENTOR,.

United States Patent Office 3,764,406 Patented Get. 9, 1973 HOT-WORKING METHOD OF PRODUCING CUBE- ON-EDGE ORIENTED SILICON-IRON FROM CAST SLABS Martin F. Littmann, Middletown, Ohio, assignor to Armco Steel Corporation, Middletown, Ohio Filed Nov. 4, 1971, Ser. No. 195,553 Int. Cl. HOlf 1/04 U.S. Cl. l48-111 8 Claims ABSTRACT OF THE DISCLOSURE A process for the production of cube-on-edge siliconiron sheet stock having uniformly good magnetic properties, comprising melting, refining, casting into slabs, hot reducing to hot rolled thin band, removing the hot mill scale, cold reducing to final thickness in one or more stages, decarburizing and recrystallizing in a wet hydrogen anneal, coating with a separator and annealing in hydrogen above about 1100 C. The cast slabs are initially hot reduced at least at a temperature of 750 to 1250 C., then heated above 1350 C. and hot rolled to a thickness of 2.5 mm. or less. The initial hot reduction produces a structure which prevents excessive grain growth during the subsequent heating and hot rolling steps.

BACKGROUND OF THE INVENTION (1) Field of the invention This invention relates to a method of producing oriented silicon-iron sheet or strip for magnetic purposes. The orientation with which the present invention is concerned is that wherein the grains or crystals are oriented in the cube-on-edge position, i.e. designated (l l0)[00l] in accordance with the Miller Indices. More particularly, the invention relates to a method of producing grain-oriented silicon-iron sheet or strip containing from about 2% to 4% silicon of uniformly excellent magnetic properties. Although not so limited, the invention has particular utility in the production of grain-oriented silicon-iron sheet or strip wherein the molten steel is strand or continuously cast into a continuous slab of thickness suitable for direct hot rolling.

(2) Description of the prior art Cube-on-edge oriented silicon-iron sheet or strip is generally made by a series of steps including melting, refining, casting and hot reducing ingots or slabs to hot rolled bands of about 2.5 mm. thickness or less. After annealing and scale removal, the hot rolled band is cold reduced in one or more stages, with intermediate anneals if necessary, to a final thickness of about 0.25 to about 0.35 mm. The strip is then usually recrystallized and decarburized at final thickness by a continuous anneal in a wet hydrogen atmosphere. Finally, the strip is coated with an annealing separator and box annealed for several hours in dry hydrogen at a temperature above about 1100 C.

As is well known, two conditions must be satisfied prior to the high temperature portion of the final box anneal in order to obtain material having a high degree of cube-ouedge orientation, viz:

(l) A suitable structure of completely recrystallized grains with a suflicient number of these grains having the final cube-on-edge orientation;

(2) The presence of inhibitors in the form of small, uniformly distributed inclusions which restrain primary grain growth in the early portions of the anneal until a vigorous secondary growth occurs during the latter, high temperature portion of the anneal.

During the above mentioned secondary grain growth portion of the anneal, the cube-on-edge grains consume other grains in the matrix having a different orientation.

The primary grain growth inhibitor, which must be present in the form of small, uniformly distributed inclusions, is usually manganese sulfide, but other inhibitors, e.g., manganese selenide, aluminium nitride, or mixtures thereof, may also be used for this purpose.

U.S. Pat. 2,599,340, issued June 3, 1952, to M. F. Littmann and J. E. Heck, discloses a process for the production of cube-on-edge silicon-iron wherein slabs rolled from ingots are heated to a temperature above about 1260 C., and particularly from about 1350 to about 1400 C. prior to hot rolling. This heating step not only prepares the metal for hot rolling but also dissolves the inhibitor present therein so that upon subsequent hot rolling the inhibitor is precipitated in the desired form of small, uniformly distributed inclusions, thereby satisfying one of the two essential conditions for obtaining highly oriented cube-on-edge material.

The practice of heating an ingot, or the product of an ingot rolled in to slab form, to a temperature above about 1260 C. and up to 1400 C. prior to hot holling has been widely adopted and is currently in use.

Strand casting into a continuous slab or casting into individual slabs of a thickness suitable for direct hot rolling is now being used for making ferrous sheet stock. The term slab as used herein is intended to include cast bodies ranging in thickness from about 10 to about 30 cm. These casting techniques are advantageous in that they avoid the loss of material from the butt and top portions of conventional ingots, which ordinarily must be cropped. For example, slabs of silicon-iron are strand cast into a thickness of 15 cm., cut to suitable length and reheated to about 1350 to 1400 C. in order to dis solve the inhibitor prior to hot rolling. However, the degree of the cube-on-edge orientation of the product of strand cast slabs has tended to be much more variable than material produced from ingots, especially across the width of the strip.

Lack of uniformity and frequent inferiority in magnetic properties of material produced by strand casting have hitherto limited the utility of strand casting despite the advantages over conventional production from ingots.

SUMMARY It has been ascertained that a principal cause of the above mentioned variable and often inferior development of cube-on-edge texture results from excessive grain size in the slab, which occurs as a result of reheating above about 1300 C. prior to hot rolling. Strand cast slabs in the as cast condition have a crystal structure which upon reheating above 1300 C. exhibits grain growth to an average diameter of about 25 mm. (about 0.5 to 1.0 ASTM grain size at 1X). By way of comparison, the average grain diameter in slabs rolled from ingots, after reheating to about 1300 C., is about 10 mm.

Accordingto the present invention, it has been found that limiting the grain size in the slabs, after reheatin to high temperature and prior to hot rolling, to a diameter not exceeding that represented by about 4.5 ASTM at 1 (corresponding to an average diameter of about 7 mm. or less) has a beneficial elfect on the development of cube-on-edge texture in the final product. Apparently the smaller grain size promotes more complete recrystallization during the processing anneals subsequent to hot rolling.

It has now been discovered that control of the grain size in the slabs after the high temperature reheating can be achieved by hot reducing (prerolling) the cast slab by at least about 5%, before reheating, at a temperature below that at which excessive grain growth occurs. Ordinarily, excessive grain growth in the slab may begin between about 1260 and 1350 C. Hence, this initial hot 3 4 reduction is carried out within the range of about 750 Percent to 1250 C., and preferably between about 850 and Carbon 0.034 1150 C. Manganese 0.062 The initial hot rolling step of the present invention re- Sulfur 0.0 24 sults in a marked reduction in the size of the grains in 5 Silicon 3.17 the slab after reheating to about 1400 C. A hot reduc- Balance substantially iron.

tion of about 25% results in a grain size after reheating of about 5 mm. average diameter (i.e. about 5-6 ASTM grain size at 1x). At least about 5% initial hot reduction is required. Reductions between about and about 10 50% are preferred.

To the best of applicants knowledge it has never previously been recognized that initial hot reduction of a silicon-iron cast slab would result in improved magnetic properties by control of grain size upon reheating to very high temperatures.

From the foregoing discussion it will be apparent that For purposes of comparison, part of the heat of Example 1 was subjected to subsequent processing in accordance with the present invention and part in accordance with conventional practice. That portion processed in accordance with the present invention was strand cast to cm. thickness, while that portion processed in accordance with conventional practice was strand cast to 15 cm. thickness. The two processes are summarized below in tabular form and designated as A and B respectively.

A B It Is a primary ob ect of the present inventlon to provide a process for the production of cube on edge i d Process of present Invention Conventional process silicon-iron sheet stock of uniformly excellent magnetic 20 1... Shear slabs into appropriate 1..- Shear slabs into appropriate properties from cast slabs. ER gi to 1 0350 lengths BRIEF DESCRIPTION OF THE DRAWINGS gg figggf 15 m 4.-. Reheat to 1.400 C. 2... Reheat to 1,400" 0.

Reference 15 made to the accompanylng drawlngs 5.-- Hot roll to 1.9 mm. thickness. 3... Hot rollto 1.9 mm.thickness.

h i 6. Strip anneal at 975 C. 4... Strip anneal at 975 C.

. 25 7.-. Cold reduce to 264mm. thick- 5 Cold reduce to .264 mm.

FIG. 1 15 a photograph at (b X magmfi l n of a ness (2 stages with interthickness (2 stages Witt: transverse section of a 15- cm. thlckness strand cast slab medlate anneal 925 lltermedlate annealat 925 in the as cast condition; 8..- Dggggbgrgze-strip danneal at 6... Dggggbgrize-strip glnneal at FIG 2 1s photograph at O'SX magmficanon of a 9... Box anneal at 1,200 O. for 24 7.-. Box anneal at 1,200 Cf ior 24 transverse section of the strand cast slab of FIG. 1 after hours in dry hydrogen. hours in dry hydrogen. reheating to about 1400 C.;

FIG. 3 is a photograph at 0.5x magnification of a Example 2 transverse section of a 20 cm. thickness strand cast slab A h t M d fi d d h of the same heat as FIG. 1 in the as-cast condition; ea was me e re me an cast t e Same FIG 4 is a photograph at 05X magnification of a ner as that of Example 1, having the following composilongitudinal section of the slab of FIG. 3 after being hot reduced 25% in thickness at 1035 C., in accordance with Percent the invention; and g 0'030 FIG. 5 is a photograph at 0.5x magnification of a 0'057 transverse section of the initially hot rolled slab of FIG. 4 q. ur 0024 after reheating to about 1400 C. S1 Icon 7".

Balance substantially Iron. DESCRIPTION OF THE PREFERRED EMBODIMENTS 7 Again, part of the heat was strand cast to 20 cm. thickness and processed further in accordance with the pres- While the present Invention is not so limited, Its preent invention (process A), while another portion was ferred embodiments will be described With reference to strand cast to 15 cm, thickness and processed further in material strand cast into continuous slabs. accordance with conventional practice (process B).

In an exemplary routing a Charge is melted in COIlVeH- The magnetic properties of the final products of the tlonal manner in an open hearth, electric arc furnace, or heats of Examples 1 and 2 produced by both processes A basic oxygen furnace, and tapped into a ladel to Which and B are compared in Table I.

1 Permeability is indieated at H=10 oersteds. 2 Core loss 15 Indicated in watts per pound at 17 kilogausses and a frequency of cycles per second. all, or a substantial part of, the required silicon is added. It will be apparent from Table I that material produced The melt may then be refined, by procedures Whi h may in accordance with the present invention exhibits a siginclude vacuum degassing if desired. The melt is next nificant improvement both in average permeability and transferred to a casting station and cast to a desired thickcore loss, and more particularly in uniformity of these ness,e.g. 15 or 20cm. properties. The permeability of material produced by Example 1 process A ranges between 1820 and 1840 with an average above 1830, as compared to permeability ranges of from A heat Was Processed 111 the above descrlbed manner 1745 to 1840 and an average of about 1804 for material and strand cast into slabs having the following composiproduced from strand cast slabs in accordance with proction: ess B.

Example 3 A heat was melted, refined and cast, in the same manner as that of Example 1, except that all the slabs were strand cast to 20 cm. thickness. The composition of the range of about cm. to about 30 cm. thickness, has little or no effect on the response to the initial low temperature hot reduction, so long as this initial reduction in thickness is at least about 5%.

From Table II it is apparent that relatively poor and heat of Examp 1e 3 was as follows P t 5 non-uniform results are obtained by the conventional proc- C b 33 ess for slabs cast to 20 cm. thickness, whereas uniform at on 0'055 and superior properties are obtained by the process of i i 16 10 A substantial number of coils has been processed to a Slhcon 7"1 final gauge of .264 mm. in accordance with the present Balance substantlauy invention, including the examples shown above, and

For purposes of comparison, part of the heat was sub- 98.4% of the test values of permeability at H=10 were jected to subsequent processing in accordance with the 1820 or higher and 62.8% of the values were 1840 or invention and part in accordance with conventional prachigher. tice. The two processes are summarized below and des- FIGS. 1-5 are photographs of etched sections of strand ignated as C and D respectively. cast slabs of Example 2 produced both by process A and process B. C D Referring to FIG. 1, which is a transverse section at Process of present invention Conventional process 0'5X magnification of a Slab cast to 15 thickness, it

v D will be noted that a columnar grain structure extends from i iii t mm appropnate Into appmpmte each surface inwardly almost to the center of the slab, 2--- Heat slabs to 1,035 C. with a relatively narrow core or band of equiaxed grains gf ggg g? to 15 at the center. FIG. 2 illustrates the elfect of reheating 4 Reheat to 1,400 0..- 2 Reheat t01,400 C. k a 25 the slab of FIG. 1 to 1400 C., in accordance with con- 33 gigf gggg ggggs 3 5??? i: ventional practice. It will be noted that excessive growth 7 Cold reduce to .264rnrh.t hick- 5.- Cohd krreducewtotitg both of the columnar and equiaxed grains has occurred, r ii t litii 22 1 52212? int rm e d iate ar iii e al at 9 25" the averag? a S126 bemg ab'out to ASTM a 8 De rbur'zesti anneal at 6 D car burizestrip anneal at magmficatlon (correspondmg to an average gram '0 852 C. in \vet h ydrogen. 825 C. in wet hydrogen. dlameter about 25 9 Box anneal at 1,200 O. for 24 7.. Box anneal at l,200 C. for 24 FIG, 3 1s a transverse ection at ()5 X ific tion hmrs dry hydrogen dry hydmgen of a slab (of Example 2) cast to 20 cm. thickness. It will be noted that the structure is substantially identical to Example 4 that of FIG. 1.

A heat was melted, refined, cast and subsequently proc- 4, Which is a longitudinal Section at 5X gessed in the same manner as Example 3 having the nification, illustrates the effect of initially hot reducing lowing composition: the slab of FIG} by 25% at a temperature of 1035 0.,

Percent in accordance with the present invention. It will be noted Carbon 31 a the columnar grain structure is still evident after hot Manganese 055 i g a d is somewhat distorted. However, a significant S lf r02 f ture of the hot rolled slab at this stage is the appear- Silicon 5 nce of numerous recrystallized grains, Which are ran- Balahee substantially iron domly dispersed throughout the interior of the slab. The

Again ah Slabs were east to 20 em thickness, with part appearance of the recrystallized grains at this stage deof the heat processed in accordance with the present in- Rends uP011 the fmperawre of hot follmg and 15 I101 vention, and another portion processed in accordance with sldered to be crltlcalconventional practice. FIG. 5 is a transverse section at 0.5x magnification The magnetic properties of the final products of the of the initially hot rolled slab of FIG. 4 after reheating heats of Examples 3 and 4 produced by both processes to 1400 C. It will be noted that reheating has resulted C and D re compared i T ble II, in the formation of a much smaller, equiaxed grain struc- TABLE II Process 0 Process D Permeability 1 N Permeability Example te ts Range Average Te ts Range Average 3 24 i,s20-1,s50 1,835 24 l,760-1,840 1,799 16 1,s10-1,s50 1,830 24 1,7851,840 1,818

Core loss 5 Core loss a. 24 .ss5. 740 .707 24 .705-. 870 .777 4- l6 665-. 735 .712 24 675-. 805 .736

1 Same as Table I.

2 Same as Table I.

Since all slabs had the same initial thickness in procture, of about 56 A grain Size at l i.e., an avere ses C d D, it ill b apparent th h h lli age grain diameter of about 5 mm. This is in marked conafter reheating (step 3 in process D) involved a greater to graif1 Structure of and iS y igreduction in thickness in the conventional process than mficant VIEW of the factthat the Slab of 2 and the corresponding Step 5 in process C in View of the fact that of FIG. 5 were each su ected to the same reheat temthat the slab has already been hot reduced to 15 cm. thickperatilre 1400 ness in Step 3 of process C. It is believed that complete recrystallization takes place 7 0 in the early stages of high temperature reheating from The data of Tables I and II show that the magnetic properties obtained from slabs reheated to very high temperature are similar for slabs cast to 15 cm. and 20 cm. thickness and processed in the conventional manner. It

nuclei formed during the initial hot rolling of the cast slab. This recrystallization structure exhibits less grain growth during the completion of the slab reheating to 1400 C. than the original cast structure as shown in is believed that initial slab thickness, at least within the FIG. 2.

It is thus apparent that application of an amount of low temperature hot working sufiicient to cause a marked reduction in the size of the grains of the slab after reheating to a temperature above about 1300 C. achieves the principal object of the present invention.

As indicated previously, the preferred range of initial hot reduction is from 10% to 50%. A 25% initial reduction was found to develop optimum grain refinement in the reheated slab. Reductions below 5% do not introduce sufficient energy to be beneficial. As percent reduction increases over 25%, the benefit as measured by grain size of the reheated slab gradually diminishes to the extent that about 50% reduction may be considered the practical upper limit of this invention. The preferred temperature range for the initial hot reduction is from 850 to 1150 C., as contrasted with the usual soaking pit temperature of 1230 C. for ingots which are rolled into slabs, which constitutes a reduction in excess of 70%.

Continuous casting installations are in operation which incorporate in-line hot reduction capability. In such an arrangement the residual heat of the cast slab may be sufiicient to permit initial hot reductions within the ranges of the present invention. This could minimize or eliminate reheating of slabs for initial hot rolling.

Although in-line hot rolling is presently being used for strand cast plain carbon steel, this results in direct recrystallization of the grain structure, unlike the situation with silicon-iron wherein the original columnar grain structure is not broken up as a direct result of hot rolling but recrystallizes during reheating to very high temperature.

The benefits of the present invention are not dependent on composition, and may be realized with any inhibitor, e.g., manganese sulfide, manganese selenide, aluminum nitride, or mixtures thereof. By way of example, the skilled worker will recognize that heats may be processed to obtain uniform magnetic properties by selecting appropriate combinations of elements within the following ranges:

Carbon 0.02-0.05

Manganese 0.04-0.12%. Sulfur 0.0l0.035%. Silicon 24%.

Nitrogen Less than 0.01%. Aluminum Less than 0.04%.

and balance substantially iron, all percentages being by weight.

As illustration, reference is made to U.S. Pats. No. 3,287,183 issued to Taguchi et al., and No. 2,867,557 issued to Crede et al., wherein two types of compositions are disclosed.

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A process for the production of cube-on-edge texture silicon-iron sheet stock containing from about 2% to about 4% silicon, comprising the steps of melting a charge of silicon-iron, casting the charge to produce a slab having a thickness of about to about 30 centimeters, heating the slab to a temperature of at least about 750 C. but below about 1250 C., initially hot reducing the slab with a reduction in thickness of 5% to re heating the slab to a temperature between about 1260 and 1400 C. to obtain a grain diameter not exceeding that represented by about 4.5 ASTM at 1 hot rolling the slab to a hot band, cold reducing to final gauge in at least one stage, decarburizing, and finally annealing under conditions which effect secondary recrystallization by causing cube-on-edge grains to consume other grains having a different orientation.

2. The process of claim 1, wherein said initial hot reducing step comprises a reduction in thickness of about 10% to about 50%.

3. The process of claim 2, wherein said hot reducing comprises a reduction in thickness of about 25%.

4. The process of claim 1, wherein said slab is heated to a temperature of from about 850 to about 1150 C.

5. The process of claim 1, including the step of strip annealing the silicon-iron after hot rolling to a hot band and before cold reducing to final gauge.

6. The process of claim 1, wherein said cold reducing step comprises cold rolling in at least two stages with an intermediate strip anneal between each stage.

7. Cube-on-edge texture silicon-iron sheet stock produced by the process of claim 1.

8. A process for theproduction of cube-on-edge texture silicon-iron sheet stock containing from about 2% to about 4% silicon, comprising the steps of melting a charge of silicon-iron, continuously casting the charge to produce a slab having a thickness of about 10 to about 30 centimeters, initially hot reducing the slab with a reduction in thickness of 5% to 50% while said slab retains the residual heat of the casting step at a temperature of at least about 750 C. but below about 1250 C., reheating the slab to a temperature between about l260 and 1400 C. to obtain a grain diameter not exceeding that represented by 4.5 ASTM at 1 hot rolling the slab to a hot band, cold reducing to final gauge in at least one stage, decarburizing, and finally annealing under conditions which effect secondary recrystallization by causing cube-on-edge grains to consume other grains having a different orientation.

References Cited UNITED STATES PATENTS 3,144,363 8/1964 Aspden et al 148-l11 3,147,158 9/1964 Fiedler 148111 3,214,303 10/1965 Fiedler 148111 3,105,782 10/1963 Walter 148111 2,599,340 6/1952 Littmann 148-111 2,473,156 6/1949 Littmann l48111 L. DEWAYNE RUTLEDGE, Primary Examiner W. R. SATTERFIELD, Assistant Examiner US. Cl. X.R. 

